A common type of printer architecture is a carriage printer, where a printhead array of marking elements is somewhat smaller than an extent of a region of interest for printing on a recording medium and a printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a media advance direction and then stopped. While the recording medium is stopped, the printhead is moved by the carriage in a carriage scan direction that is substantially perpendicular to the media advance direction as marks are controllably made by marking elements. After the printhead has printed a swath of an image while traversing the recording medium, the recording medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
One example of a carriage printer is an inkjet printer. An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors that function as marking elements, each ejector consisting of an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator can be one of various types, including a heater that vaporizes some of the ink in a pressurization chamber in order to propel a droplet out of an orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the printhead is moved relative to the recording medium.
In order to produce high quality images, it is helpful to provide information to the printer controller electronics regarding the printing side of the recording medium and the characteristics of the marks printed on the recording medium by the printhead. Information about the recording medium itself can include whether it is a glossy or matte-finish paper. Information about the marks printed on the recording medium can include relative alignment between marks of different colors, angular misorientation of the printhead relative to the direction of relative motion of the recording medium, or relative alignment of marks between left to right and right to left passes in a carriage printer, or missing marks corresponding to defective portions of the printhead, such as bad nozzles in an inkjet printhead. Using the information from the optical sensor, the printer controller is designed to control the printing process to optimize printing quality by using appropriate print modes for the detected media type, by correcting for various types of misalignments and by compensating for defective portions of the printhead.
It is known in the prior art to attach an optical sensor assembly to the printhead carriage of a carriage printer. See for example U.S. Pat. Nos. 5,170,047; 5,905,512; 5,975,674; 6,036,298; 6,172,690; 6,322,192; 6,400,099; 6,623,096; 6,764,158; 6,905,187 and 7,800,089. Such an optical sensor assembly can be called a carriage sensor. In the same way that the printhead can mark on all regions of the paper by the back and forth motion of the carriage and by the advancing of the recording medium between passes of the carriage, it is desirable for the carriage sensor to be able to provide optical measurements, such as optical reflectance, for all regions of the paper. A carriage sensor assembly typically includes one or more photosensors and one or more light sources, such as LED's, mounted such that the emitted light is reflected off the printing side of the recording medium, and the reflected light is received in the one or more photosensors. LEDs and photosensors can be oriented relative to each other such that the photosensor receives specular reflections of light emitted from an LED (i.e. light reflected from the recording medium at the same angle as the incident angle relative to the normal to the nominal plane of the recording medium) or diffuse reflections of light emitted from an LED (i.e. light reflected from the recording medium at a different angle than the angle of incidence). A carriage sensor such as that described in U.S. Pat. No. 7,800,089, which is incorporated by reference herein in its entirety, works well for detecting surface roughness of the recording medium, side edges of the recording medium, and alignment marks and test patterns printed on the recording medium. However, a more compact carriage sensor is needed for printers having a small footprint. It has been found that for some carriage printers having a reduced width that a carriage sensor such as the one described in U.S. Pat. No. 7,800,089 cannot detect both side edges of the widest compatible recording medium without interfering with other portions of the printer. In addition, a more economical carriage sensor is needed, especially for low cost printers. It is also desirable to have a carriage sensor assembly that is more efficient than carriage sensors of the prior art.